The field of this invention relates to the liquid-phase oxidation of p-acetoxyacetophenone to p-acetoxybenzoic acid in high yield by the addition of acetic anhydride during the reaction, thus increasing the yield of reaction product by about 35% to about 40%. This invention also relates to the addition of acetic anhydride to the reaction as a reaction promoter to increase the yield of p-acetoxybenzoic acid, thus obtaining a more economical and efficient process. This invention also relates to the production of p-acetoxybenzoic acid by continuous, semicontinuous, or a batch process. The batch process is one where p-acetoxyacetophenone is introduced into the process at the beginning of the process and product is removed at the end of the process. The semicontinuous process is one where p-acetoxyacetophenone is added at one or more times during the course of the process, as well as at the beginning of the process, and product is removed at the termination of the process. The continuous process is one where p-acetoxyacetophenone is introduced into the process at the beginning of the process, as well as during the course of the process, and product is removed throughout the process.
The possibility of using liquid-phase instead of vapor-phase oxidation for the preparation of benzene carboxylic acids was first indicated by the disclosure in U.S. Pat. No. 2,245,528 of the catalysis provided by transition of variable valence metals, especially cobalt, in a liquid phase of saturated lower aliphatic acid at temperatures from 100.degree. C. to 320.degree. C. and pressures to maintain the liquid phase of the aliphatic acid. Such catalysis, according to said patent, was advantageously promoted by the use of a ketone, such as methylethyl ketone, or an aldehyde, such as acetaldehyde. Unfortunately, such aldehyde or ketone promoted variable valence metal catalysis was useful only for converting mono-, di- or trimethylbenzenes to their respective benzene monocarboxylic acids: benzoic, toluic and dimethyl benzoic acids. Two separate, later, and somewhat parallel lower temperature (80.degree. C.-100.degree. C.) modifications of the aldehyde or ketone promoted cobalt catalysis in liquid phase of acetic acid did provide commercially feasible conversion of xylenes to phthalic acids, especially p-xylene to terephthalic acid, but only at the expense of using rather high concentrations of cobalt with respect to p-xylene.
The disadvantages of using high concentrations of cobalt promoted with large quantities of aldehyde or ketone were overcome and, at the same time, a greater choice of variable valence metal oxidation catalysts was made available and a wider choice of alkyl-substituted benzene starting materials for benzene di-, tri- and higher carboxylic acids was provided by the discovery of the unique promotional effect on said variable valence metal by bromine ion, provided per se or formed in situ with or without acidic reaction medium provided by C.sub.1 -C.sub.8 monocarboxylic acids having no hydrogens on a tertiary carbon, such as benzoic acid and the saturated aliphatic monocarboxylic acids, preferably acetic acid. Such bromine-variable valent metal catalysis was first disclosed in U.S. Pat. No. 2,833,816.
The bromine-polyvalent metal catalysis in acetic acid solvent has been in commercial use in many countries for the manufacture of benzene carboxylic acids and derivatives of benzene carboxylic acids for many years, such as terephthalic acid from p-xylene. However, for example, in the absence of acetic acid solvent, best yield of a single phthalic acid (e.g., terephthalic acid) on a once-through basis of the xylene, amounted to about 20 weight percent (12.8 mole), according to U.S. Pat. No. 2,833,816. According to U.S. Pat. No. 3,920,735 the Mn-Br and Co-Mn-Br catalyst system is improved by the addition of zirconium. However, not mentioned, but illustrated in Tables I, II and IV in U.S. Pat. No. 3,920,735, is the fact that, when part of the zirconium is added, combustion of the feedstock to carbon dioxide increases.
The preparation of acyloxy aromatic carboxylic acids by oxidation of acyloxy aromatic ketones with oxygen in the presence of a catalyst and coreductant has been taught. According to EP Patent Application No. 170483, preparation of acyloxy aromatic carboxylic acids is accomplished by oxidation with oxygen of acyloxy aromatic ketones R--C(O)O--Ar--C(O)R employing a transition metal catalyst and coreductant where Ar is a divalent aromatic radical, preferably 1,4-phenylene, R is preferably the same or it may be different and contain from 1 to 18 carbon atoms. The acyloxy aromatic ketone, 4-acetoxyacetophenone and its oxidized product, 4-acetoxybenzoic acid, is prepared. The claimed catalyst and coreductant are manganese ions and acetaldehyde, respectively.
The preparation of p-acetoxybenzoic acid from p-cresyl acetate with oxygen in the presence of bromine and a catalytic mixture comprising a cobalt compound and a manganese compound has been disclosed. However, yields are low and the aldehyde is produced in quantity. According to Japanese Patent No. SHO 50-35066, in Table II, in the oxidation of p-cresyl acetate with oxygen in the presence of a bromine compound, cobalt acetate and manganese acetate wherein acetic acid and acetic anhydride were employed as solvents, yield of acetoxybenzoic acid was 42.3 mole % and yield of acetoxybenzaldehyde was 12.4 mole %. In the absence of acetic anhydride, yield of acetoxybenzoic acid was 39.6 mole % and yield of acetoxybenzaldehyde was 21.2 mole %. In the absence of acetic acid, yields dropped to 12.6 mole % and 7.2 mole %. Unreacted feed material, moreover, was sizeable, ranging from 37.5 mole % in the absence of acetic anhydride, to 55.8 mole % in the absence of acetic acid, and being 42.1 mole % when both solvents, acetic acid and acetic anhydride were present.
Despite earlier evidence, as disclosed in U.S. Pat. No. 2,245,528 and EP Patent Application No. 170483, that the presence of a ketone was useful as a promoter in the oxidation of mono-, di-, or trimethyl benzenes to their respective monocarboxylic acids, or a coreductant was useful in the oxidation of acetoxyacetophenone to acetoxybenzoic acid in the oxidation of the respective feed materials with oxygen in the presence of transition metals, especially cobalt, it has been found unexpectedly that liquid-phase oxidation of an acetoxylated benzylic ketone to an acetoxybenzene monocarboxylic acid can be obtained in advantageous yields in a solvent of a lower aliphatic saturated carboxylic acid in the presence of a catalyst mixture comprising a mixture of transition metals, a bromine compound, and alternatively, a zirconium compound, and a promoter consisting essentially of an anhydride of the same carboxylic acid used as solvent.